Online Documentation for SQL Query for PostgreSQL
In order to build a diagram in Diagram View, first of all you should select the source field(s) to be represented in the diagram.
Note that only fields of numeric types can be used in a diagram, and each selected field is represented by a separate diagram series. Use the Diagram Control panel on the left to select the fields for your diagram. If the Field Name combo-box is empty then either the data source is not yet selected or it contains no numeric fields.